The present invention relates to a monolithically-integrated optical device and in particular, to an integrated optical planar waveguide amplifier. The invention also relates to a method of fabricating such an optical device.
Optical amplifiers are an important component in optical networks for distributing optical signals. In recent years, erbium-doped optical fibres have been developed which have the capability of amplifying an optical signal. In order to amplify an optical communications signal propagating in an erbium-doped fibre amplifier, light of a different wavelength is coupled into the fibre from a pumping laser. The pumping laser stimulates electronic transitions which amplify the communications signal as it passes through the erbium-doped optical fibre.
In applications where optical components need to be relatively small and device integration is desirable, it is advantageous to provide an optical amplifier in the form of a planar waveguide integrated on a single substrate. However, there are difficulties in integrating erbium-doped amplifiers. In particular, since the longitudinal dimensions of integrated amplifiers tend to be much smaller than the longitudinal dimensions of erbium-doped fibre amplifiers, it is necessary to increase the gain of the amplifier. Attempts have been made to increase the gain by increasing the percentage of erbium. However, the gain in erbium-doped fibre amplifiers has been found to decrease when the erbium doping concentration exceeds a critical level. For example, in silica-based amplifiers, maximum gain is achieved with an erbium concentration of around 0.01-0.02 atomic %. It is believed that at higher concentrations of erbium, the gain is reduced due to an increase in erbium-erbium interactions. One method of addressing this problem has been to increase the solubility of erbium in silica-based glass by incorporating various glass modifiers into the structure, such as sodium and calcium. However, this approach has had limited success, particularly when the core layer of the waveguide is deposited by sputter deposition.
Sputter deposition involves bombarding a target of source material in a manner which ejects electrons and target atoms from the target and deposits at least some of the ejected target atoms onto a substrate. In configurations where a magnet is positioned beneath the target so as to increase plasma densities closer to the target, the technique is referred to as magnetron sputtering. One of the characteristics of sputter film deposition is that different species of target atoms tend to have different deposition rates due to differences in gas scatter rates and substrate sticking coefficients. Thus, a film deposited from a composite target containing a number of different atomic species, (e.g. silica, erbium, sodium and calcium) can have a composition which is different to that of the composite target. There is therefore a need for an integrated optical planar waveguide amplifier which has an improved gain, and for an improved method of fabricating a planar optical device in which an amplifier is integrated with other optical devices.
In accordance with a first aspect of the present invention there is provided an integrated optical device comprising a metaloxide-based optical planar waveguide amplifier monolithically integrated on a common substrate with at least one additional planar waveguide selected from a group comprising:
(i) a planar waveguide signal-processing circuit arranged to process an optical communications signal; and
(ii) a planar waveguide pump-signal coupling circuit arranged to couple or decouple a pump wavelength to or from the amplifier;
wherein the amplifier has a metal-oxide-based core comprising an optically-transmissive metal oxide material doped with a gain medium and is arranged to amplify an optical communication signal when optically pumped with a source of pump radiation.
Preferably, the metal oxide comprises at least 50 mol % of the core of the amplifier, and more preferably at least 70 mol % of the core. The composition of the core of the amplifier may predominantly comprise aluminium oxide. In one embodiment, the metal oxide comprises at least 80 mol % of the core. The amplifier may be formed directly on the substrate. Alternatively, one or more additional layers, such as another planar waveguide, may be interposed between the amplifier and the substrate.
Preferably, the amplifier is integrated with and coupled to at least the planar waveguide signal-processing circuit. The planar waveguide communications-signal-processing circuit may comprise one or a combination of planar waveguide devices selected from a group comprising:
a communications-signal multiplexer, arranged to multiplex a plurality of communications wavelengths;
a communications-signal demultiplexer, arranged to demultiplex a plurality of optical communications wavelengths;
a channel gain equaliser;
an Nxc3x97M optical switch matrix;
an optical modulator;
an optical attenuator;
a variable optical attenuator;
an add-drop multiplexer; and
a reconfigurable add-drop multiplexer.
The communications-signal multiplexer and the communications-signal demultiplexer may comprise an arrayed-waveguide grating. Any number of other optical components may be monolithically integrated on the common substrate with above-described planar waveguides. The pump-signal coupling circuit may comprise one or more planar waveguides selected from a group comprising:
a pump-signal multiplexer arranged to multiplex an optical communications wavelength and an optical pump wavelength; and
a pump-signal demultiplexer arranged to demultiplex an optical communications wavelength and an optical pump wavelength.
The pump-signal multiplexer and the pump-signal demultiplexer may each comprise an asymmetric Mach-Zehnder inferometer.
The pump-signal coupling circuit may incorporate a slab waveguide for collecting pump radiation from a plurality of sources and guiding the radiation towards the amplifier. In one embodiment, the pump-signal coupling circuit is arranged to be pumped by a laser diode bar array. The integrated optical device may firer comprise a laser diode bar array integrated on the common substrate and optically coupled to the planar waveguide amplifier. Pump radiation may be coupled into the amplifier using a cladding mode of the amplifier. In an alternative embodiment, the planar waveguide coupling circuit is arranged to be optically connected to an external pumping source not integrated in the optical device
The amplifier may be arranged to provide sufficient gain (communications sight amplification) to compensate for insertion losses of the integrated optical device. For example, the amplifier can be arranged to compensate for any optical losses in the signal-processing circuit and pump-signal coupling circuit or in optical connections between those circuits. In one embodiment, the signal-processing circuit comprises a communications-signal demultiplexer and the amplifier is arranged to compensate for any optical losses arising from the demultiplexer.
The amplifier may comprise one or more amplifiers, and may be arranged to amplify communications signals in one or more respective input channels of the communications-signal processing circuit. Alternatively, the amplifier may comprise one or more amplifiers arranged to amplify communications signals in one or more respective output channels of the signal-processing circuit.
The amplifier may be arranged on the substrate adjacent and substantially parallel to a side of the signal-processing circuit so as to conserve space on the substrate. The amplifier may include at least one mirror structure for reflecting and guiding optical signals. The mirror structure can be used to guide a signal around a sharp comer rather than a curved bend, and can thus enable the amplifier to be laid out over a smaller area than is possible if smooth bends are used. In one embodiment, the mirror structure operates by means of total internal reflection at an air interface. The air interface may comprise an air interface of a trench etched into the surface of the device. In another embodiment, the mirror structure comprises a metallised surface, such as a metallised wall of a trench. In a further embodiment, We mirror structure comprises a Bragg grating.
The amplifier may include an optical coupling region for optically coupling a terminal end of the amplifier to another planar waveguide. Where the amplifier is to be coupled to a waveguide of different effective refractive index, the coupling region may be formed such that there is a gradual transition of effective refractive index from the terminal end of the amplifier to the other waveguide, so as to minimise coupling losses. Preferably, the transition of effective refractive index is adiabatic. In the coupling region, a core of the amplifier may be tapered in cross-sectional dimensions and/or refractive index so as to create a substantially adiabatic transition of effective refractive index from the amplifier to the other waveguide. In one embodiment, the amplifier projects into a second core to which the amplifier is to be coupled. The second core may comprise a core of the signal-processing circuit or the pump-signal coupling circuit.
The signal-processing circuit and the pump-signal coupling circuit may comprise silica-based waveguides. The gain medium may comprise lanthianide atoms such as erbium or ytterbium. In one embodiment, the gain medium comprises both erbium and ytterbium. The inventors have found that the solubility of erbium in aluminium oxide is greater than in silica-based materials. Thus, where the amplifier core comprises erbium-doped aluminium oxide, the present invention has the advantage of enabling higher core concentrations of erbium to be attained than is possible with silica-based cores. For example, the concentration of erbium in the core may be in the range from about 0.1 atomic % to about 1.0 atomic %. By comparison, known silica-based amplifying cores are doped with a maximum of about 0.01 to 0.02 atomic % of erbium.
The core may optionally include a gain-broadening dopant for broadening a gain spectrum of the amplifier. For example, the gain-broadening dopant may comprise, but is not limited to, fluorine, tellurium, sodium or calcium. Also, the core may optionally comprise at least one refractive-index-modifying dopant, such as fluorine. Further, the core may optionally comprise a dopant capable of reducing interactions between atoms of the gain medium which decrease the potential gain of the amplifier. For example, where the gain medium comprises erbium, the core may be doped with fluorine to reduce erbium interactions and to decrease the refractive index of the core. An advantage of using fluorine is that it tends to scavenge hydroxyl species from aluminium oxide. Hydroxyl species are believed to absorb energy from erbium ions.
Preferably, the metal-oxide-based core is encapsulated between an optical buffer layer below the core, and an optical cladding layer above the core, both of which have a lower refractive index than the core. The cladding layer and/or buffer layer may be silica-based. The cladding layer and/or buffer layer may comprise a silica-based core of another waveguide.
The amplifier core may comprise a sputter-deposited material. Preferably, the core comprises a film of material in which any defects which could potentially cause absorption at a wavelength of the optical signal have been substantially eliminated as a result of using appropriate deposition conditions and/or post-deposition annealing. Preferably, the core comprises a film in which any defects and impurities which could potentially cause non-radiative energy transfer from excited erbium atoms have been substantially eliminated as a result of using appropriate deposition conditions and/or post-deposition annealing.
In accordance with a second aspect of the present invention, there is provided a method of fabricating an optical device comprising a plurality of planar waveguides monolithically integrated on a common substrate, the method comprising:
forming a planar waveguide amplifier on the substrate, the amplifier having a metaloxide-based core comprising a metal oxide doped with a gain medium and being arranged to amplify an optical communications signal when optically pumped with a source of pump radiation;
forming at least one additional integrated planar waveguide on the substrate, the additional waveguide being selected from a group comprising:
(i) a planar waveguide communications-signal processing circuit arranged to process an optical communications signal; and
(ii) a planar waveguide pump-signal coupling circuit arranged to couple a pump wavelength into the amplifier.
The step of forming the amplifier may comprise;
depositing a core layer; and
using lithographically-defined etching to shape the core layer into the metal-oxide-based core with a channel geometry.
The step of forming the amplifier may further comprise forming an optical buffer layer on which the core is subsequently formed, and forming an optical cladding layer over the buffer layer and core. The buffer layer and/or cladding layer may comprise a core of a silica-based waveguide. For example, the core of the amplifier may be disposed within a core of a silica-based waveguide. The core of the amplifier may be annealed post deposition, and may be annealed again after being etched. The annealing may be carried out using annealing conditions selected to substantially eliminate defects which could potentially cause absorption at a wavelength of an optical signal to be amplified. The annealing conditions may also be selected to substantially eliminate defects and impurities which could potentially cause non-radiative energy transfers from excited atoms of the gain medium. Where each additional planar waveguide has a silica-based core, the metal-oxide-based core of the amplifier is preferably formed before each silica-based core is formed in order to minimise the fabrication temperature of the silica-based core.
Where the additional planar waveguide comprises the communications-signal processing circuit, the amplifier may be formed on the substrate adjacent and substantially parallel to the signal-processing circuit.
The buffer and cladding layers of the amplifier and each additional planar waveguide may be silica-based. The core of each additional planar waveguide may also be silica-based.
Preferably, the step of forming the amplifier further comprise forming a coupling region at a terminal and of the amplifier for coupling optical signals between the amplifier and another waveguide. The step of forming the coupling region may comprise forming the core of the amplifier such that there is a gradual transition of effective refractive index from the terminal end of the amplifier to the other waveguide. The core of the amplifier may project within a second core to which the amplifier is to be coupled. The gradual transition of effective refractive index may be achieved by forming the core such that the cross-sectional dimensions and/or refractive index of the core are adiabatically tapered. The second core may comprise a core of the signal-processing circuit or the pump-signal coupling circuit.
The gain medium in the amplifier core may comprise lanthanide atoms such as erbium and/or ytterbium. The amplifier core may be in the form of a channel waveguide.
The core of the amplifier may be formed from a metal-oxide-based core layer which may be deposited by sputtering. Preferably, the core layer is deposited by reactive DC sputtering, and most preferably, by reactive DC magnetron sputtering. Both forms of DC sputtering have the advantage of producing a higher deposition rate than is possible with RF sputtering. Where the expression xe2x80x9cDC sputteringxe2x80x9d is used herein. it is to be understood to include both magnetron DC sputtering and non-magnetron DC sputtering, unless otherwise specified. The sputtering may be carried out in an atmosphere which comprises oxygen and an additional reactive gas capable of modifying the refractive index of the core layer. The additional reactive gas may contain fluorine, and may comprise carbon tetrafluorine (CF4). Where the reactive gas contains fluorine, the refractive index of the deposited core layer may be reduced through a formation of aluminium oxyfluorine. A sputtering target used to deposit the core layer may be fluorinated during the deposition to an extent sufficient to increase a sputtering rate of the target. Alternatively, or in addition, the core layer may be fluorinated directly as it grows. The fluorination may be carried out by cyclically flowing a fluorine-containing gas over the target or core layer and then halting the flow of the fluorine-containing gas in a manner which prevents total fluorination (i.e. conversion to AlF3) of the entire core layer. Thus, the core layer may be in the form of a multilayered structure in which the concentration of fluorine changes from layer to layer, preferably periodically, resulting in an average refractive index which is lower than that of pure aluminium oxide.
The core layer may also be formed by simultaneously DC sputtering two targets of aluminium oxide, only one of which is fluorinated, and forming a core layer composed of material sputtered from both targets. Preferably, the substrate is exposed to only one of the two targets at a time, for example by cyclically moving the substrate from one target to the other using a substrate rotation stage. Alternatively, the core layer may be formed by simultaneously exposing the substrate to both targets. Preferably, the core layer is deposited by reactively DC sputtering at least one metallic target containing aluminium in a sputtering atmosphere containing oxygen. The atmosphere during the reactive DC sputtering may further comprise a noble gas such as argon
The reactive DC sputtering is preferably carried out such that there is a level of ion bombardment at the target surface which is sufficient to prevent surface passivation (i.e. oxide formation) of the gain medium in each target. Where the sputtering atmosphere comprises oxygen mixed with a noble gas such as argon, the gas flows of oxygen and the noble gas are preferably arranged so as to reduce or prevent oxidation of the exposed target surface. In one embodiment, a noble-gas outlet is provided close to the target surface and an oxygen outlet is provided closer to the substrate surface, so as to minimise the concentration of oxygen at the target surface.
Any silica-based layer in the amplifier and additional waveguide(s) may be deposited by plasma-enhanced chemical vapour deposition (PECVD), preferably in the absence of nitrogen or nitrogen-containing gases. A silica-based layer which is deposited in this way has the advantage of exhibiting reduced optical absorption in the wavelength range from 1.50 xcexcm to 1.55 xcexcm due to the absence of a nitrogen-induced absorption peak in this region. A liquid source of precursor may be used in the plasma-enhanced chemical vapour deposition to form each silica-based layer. The liquid source of precursor may comprise tetra ethyl oxysilane.
For the purposes of this specification it is to be clearly understood that the word xe2x80x9ccomprisingxe2x80x9d means xe2x80x9cincluding but not limited toxe2x80x9d, and that the word xe2x80x9ccomprisesxe2x80x9d has a corresponding meaning.
Embodiments of invention will now be described, by way of example only, with reference to accompanying drawings.